The present invention relates generally to identifying and regulating the concentration of nitrogen buildup in an operating fuel cell, particularly to controlling the bleed of nitrogen in an anode loop of a flow shifting fuel cell system, and more particularly to simplifying a bleed algorithm for a flow shifting anode flowpath to maximize stable average stack voltages while minimizing hydrogen loss during the nitrogen bleed.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, an electrolyte in the form of a membrane is sandwiched between the anode and cathode to produce a layered structure commonly referred to as a membrane electrode assembly (MEA). Each MEA forms a single fuel cell, and many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output.
One fuel cell configuration that is particularly useful is referred to as a flow shifting fuel cell system. In such a system, two (or more) stacks have their respective hydrogen (or other fuel) flowpaths fluidly coupled to one another in series such that ports that allow the flow of fuel to and from each stack can function as both fuel inlet and outlet, depending on the flow direction of the shifted fuel. The system gets its name from the serially-plumbed anode flowpath between the two stacks, as fuel flows back and forth between the stacks in a semi-closed cyclical pattern. In this way, while the anode flowpath of one of the stacks is accepting fuel into its anode flowpath, the other can be closed off (i.e., dead-ended) to prevent the escape of the fuel that has passed through the stacks. After a certain period, a combination of valves or related flow manipulation devices cause a switch in flow direction, and the role of the two stacks reverses such that reactant flows from the second stack and into the now dead-ended first. In this way, the fuel is shuttled back and forth between the two anode flowpaths, while fresh fuel can be added to the stack that is not being dead-ended. Flow shifting fuel cell systems have advantages over other approaches, such as anode flowpath recirculation-based systems, for while both can be used to improve the hydration of anode flowpaths and the electrolytes, the recirculation-based system does so with recirculation pumps and other heavily-burdened components that, in addition to increasing system cost, weight and complexity, can wear out, thereby subjecting the system to greater maintenance concerns. In addition, the use of such pumps requires a source of power (for example, electrical power) that, being supplied by the operation of the fuel cells, reduces overall system efficiency.
As with most MEA fuel cell systems that react hydrogen and air across a membrane, the operation of a flow shifting fuel cell system causes a depletion of the oxygen present in the cathode flowpath, leaving behind unreacted nitrogen. The diffusion of this nitrogen across the membranes of the individual fuel cells and into the anode flowpath contributes to the dilution of hydrogen fuel. Such nitrogen build-up within the relatively closed anode flowpath can lead to decreased stack voltage, which in turn decreases power output and stack efficiency. To meliorate the effects of nitrogen dilution, bleed valves are placed within each stack's anode flowpath to vent or purge the nitrogen-rich gas therein. One way to do this is to bleed constantly, which entails leaving the bleed valves to alternate between open and closed all the time. Unfortunately, this is highly inefficient and would end up dumping otherwise useable hydrogen fuel overboard as well. In another approach, the valves are periodically opened at select intervals to allow venting to the atmosphere of the nitrogen and other reaction by-products without the inefficiencies introduced by the constant opening and closing of the continuous approach. Nevertheless, this approach is disadvantageous in that if too long of a period goes by before the bleed valves are opened, unstable operating conditions can arise due to an impermissibly low concentration of hydrogen.
It is possible to employ a proactive (i.e., predictive) trigger-based approach that tells the system that it is time to bleed. Such a trigger would initiate a bleed sequence before an adverse performance or operability issue arises, such as a reduction in voltage during operation. One example of a proactive trigger could be a nitrogen crossover model that predicts how much nitrogen has built up between the stacks. For example, if the model prediction states there is eighty one percent hydrogen between the stacks and the threshold to start bleeding is eighty percent, then the bleed could be triggered after the model prediction falls from eighty one percent to eighty percent. This would presumably occur before any of the stack voltages started oscillating with the shift period. This approach is disadvantageous in that there is an inherent amount of predictive uncertainty, especially as the stack ages and develops minute holes that permit greater crossover.
Still another example of a proactive trigger would be to have a constant bleed through a fixed orifice, where the nitrogen crossover rate can be approximated; the bleed orifice is sized to have the bleed rate equal the crossover rate. This approach has the advantage of being simple (as it does not require a valve), but suffers from the same problem mentioned above in that as the nitrogen crossover rate changes over time (due to, for example, the development of pinholes in the stack), the orifice may be too small to exhaust the built-up nitrogen quickly enough to keep up with the crossover rate.
In another proactive approach to tell the system that it is time to bleed, one or more direct hydrogen measuring sensors (such as a thermoconductivity device) can be placed between the stacks. Unfortunately, the anode flowpath is a harsh, wet environment, and finding a direct hydrogen measuring sensor that reliably, rapidly and accurately operates in such an environment is difficult and expensive. Even if such a system were employed, it would be desirable to have a back-up capability included.
Accordingly, it is desirable that a flow shifting fuel cell system provide the operability enhancements made possible through the use of bleed valves that minimizes system complexity and efficiency impacts. It is further desirable that a system remove nitrogen from an anode flowpath while minimizing the purging of unused hydrogen. It is still further desirable that a system and method of operation does not rely on a predictive model, instead employing an actual feedback of the performance of the stack. It is yet further desirable that such an approach is relatively impervious to changes in stack performance due to aging or the like.